The present invention relates to an inverter.
Japanese Laid-Open Patent Publication No. 2009-60776 discloses an electric motor controlling method that increases the temperature of a capacitor of an inverter device when the ambient temperature is low. Specifically, depending on the ambient temperature of the inverter device, a ripple voltage is generated by the equivalent series resistance of the capacitor. The maximum allowable motor current value at which the components of the inverter device are not damaged by the ripple voltage is set using a map or a relational expression representing a relationship between the maximum value of the motor current and the temperature. In an temperature range in which the set maximum allowable motor current value is lower than the value capable of generating torque required for starting the electric motor, the position of the rotor of the electric motor is estimated and a direct current lower than or equal to the maximum allowable motor current value are supplied as a d-axis current to increase the temperature of the capacitor of the inverter device. After the temperature of the capacitor reaches and exceeds a temperature at which the maximum allowable motor current value is capable of generating the torque for starting the electric motor, the inverter device is controlled to supply alternating currents to the electric motor.
In an electric motor and an inverter device that are used under a low-temperature environment, even if the electric motor is normally started after a warm-up control is performed to sufficiently increase the temperature of the capacitor of the inverter device, the inverter device will be under a low-temperature environment when the electric motor is stopped or temporarily stopped. As a result, the temperature of the capacitor will be lowered again. Also, for smoothly starting an electric motor under a low-temperature environment, warm-up control for increasing the capacitor temperature needs to be smoothly shifted to normal operation control for normally driving the electric motor.